The performance of modern airborne radars in a high clutter environment is known to be limited by the "flicker" noise associated with the spectrum of even the highest quality solid state frequency sources. The radar degradation is due to the phenomenon known as "clutter spreading" wherein the spectrum of the received clutter is effectively widened by the flicker modulations on the receiver local oscillator signal. The widened clutter spectrum then overshadows small desired targets in adjacent doppler bands thus preventing their detection by the radar. This continues to be a serious problem in airborne radars since the performance of other system components has advanced to the point where the noise performance of the microwave frequency source presents a basic limitation to system performance in a clutter environment.
Herein described is a frequency source wherein the flicker noise is essentially eliminated, thereby allowing system requirements to be met with adequate margin and without the need for carefully screened components.
There is described a parametric crystal oscillator which can be frequently multiplied to the desired microwave frequency for radar applications. However, the techniques described for realizing low noise oscillation are by no means limited to the crystal oscillator application but are applicable to other types of oscillators. In particular, the techniques can be applied to cavity stabilized microwave oscillators where the problem of flicker noise and of voltage tuning without noise degradation are severe.
The general mechanisms which determine oscillator phase noise are well understood. The phase noise of a device is commonly described by the quantity L(F) which is defined as the ratio of the phase noise power on one side of a carrier, at frequency F removed from the carrier, to the carrier power, per unit band-width. Usually L(F)&lt;&lt;1.
In general, oscillators consist of a frequency determining resonator and some amplifier or negative resistance device known as a sustaining circuit. The sustaining circuit must employ some means to limit the amplitude of the oscillation to a steady state level. This is accomplished by some nonlinearity in the sustaining circuit. In circuits involving the transfer of power between (not necessarily harmonically related) frequencies the oscillation amplitude can be a function of the available power at some input or "pump" frequency.
The important considerations regarding the noise performance of an oscillator are: the inherent noise of the sustaining circuit; the bandwidth of the loaded resonator; and the amount of excess gain or excess negative resistance of the sustaining circuit and its behavior in the limiting process.
There exists phase noise in a high quality transistor crystal oscillator designed for doppler radar applications. The sustaining circuit contains both flicker and thermal noise sources. The thermal noise represents an effective noise figure and is additive in nature so long as the nonlinearities in the crystal and sustaining circuit are small. The flicker noise component decreases at 10 db/decade and is multiplicative in that its ratio of signal power to carrier power is essentially independent of carrier power. It has been observed that the flicker noise decreases from about -115db.sqroot.HZ at 1 HZ for many devices. This observation has been verified many times although there has been success at reducing its effects by the application of video and RF negative feedback.
It is well known that the oscillator phase noise is that of its sustaining circuit modified by a function. (The thermal noise of the resonator losses are lumped in with the sustaining circuit noise.) Above the half bandwidth of the loaded resonator the oscillator noise is that of the sustaining circuit, while below the half bandwidth the oscillator noise rises at 6 db/octave faster than the sustaining circuit with decreasing frequency.
The phase spectrum of the transistor crystal oscillator shows that unless the thermal noise is very large the low frequency phase noise of the transistor crystal oscillator is determined by the sustaining circuit flicker noise and the loaded resonator bandwidth. This low frequency flicker noise remains as a basic problem with the transistor crystal oscillator and thus with the clutter performance of airborne radars.
The desirable properties of a low noise oscillator are then: its flicker noise should be small or nonexistent; the circuit should not significantly degrade the resonator bandwidth; and the ratio of signal power to thermal noise should be large. A new circuit configuration having these desirable properties is the object of this invention.
The varactor diode finds many applications in RF circuitry. Its uses include frequency multipliers and dividers, parametric amplifiers, frequency converters, the voltage tuning of resonators, and other applications.
An observed property of the varactor diode is that under certain conditions of bias and RF drive, it exhibits little or no flicker noise.
The invention described herein utilizes a varactor diode to form a parametric crystal oscillator which is essentially free of flicker noise along with means to derive large useful power from the parametric oscillator without the introduction of substantial flicker noise and without introducing substantial degradation of the resonator bandwidth.